I hate to be all Yoda about this...but...you just kinda know :)
If what you're doing has an obvious physical response, one you can SEE and say, hey, it worked...then you SEE it. But it's not always that simple. Sometimes it's a change in the way you feel, sometimes the answer unexpectedly leads you to something else in your life that you can do to achieve the desired results (for example...last year, I did this money spell, to get a raise at work (coinciding with actually asking my boss for a raise)..I was flat broke, my bills were piling up, and I REALLY needed more money. I thought. I was denied the raise (looong story about the evils of working for IBM there), but was led to discover that I had all the money I needed, I just wasn't using it right! I re-budgeted and kept detailed logs of where I was spending, and I came up with a great plan. I still make the same amount, and now I have enough to spend and save, as well as take care of everythihg).
Distinguishing your results from coindidence...hehe, you just know. I know in my case, it just feels different when it's a plain old coincidence. A lot of that knowing comes from your own self confidence and intuition.
Some people say, that if you really want a tangible "sign" that a working has come to pass, that you should ask the god/dess/es specifically to be shown a sign, while you're doing the working. I've never asked for one, but folks who have say they do recieve them.
Hope this helps...I certainly babble on, don't I? :)